This invention relates generally to electronic timepieces having an analog time display whose hands are operated by a pulse-driven stepping motor, and more particularly to a timepiece of this type which includes a system adapted to effect rapid correction of reading errors in the order of plus or minus 30 seconds or less.
Electronic watches are known which display time in analog form by means of hour, minute and seconds hands that are advanced by a stepping motor driven by low-frequency pulses derived by a multi-section frequency divider from a high-frequency crystal-controlled time base or frequency standard. Though modern watches of this type are highly accurate, after one or more weeks of continuous operation, they usually deviate plus or minus a few seconds from the precise time.
With a view to effecting a fast correction for relatively small deviations in the seconds reading of such electronic watches, Swiss Pat. No. 556,055 and U.S. Pat. No. 3,967,442 disclose a correction system for this purpose which includes an actuating button or element accessible to the user. This element, when actuated, causes the seconds hand to assume its proper position. The system is provided with a reference counter that is coupled to the output of the frequency divider and is resettable by the actuating element. Also provided is a synchronous counter, this counter and the stepping motor for the display being supplied through a logic circuit with pulses produced by the divider whereby the synchronous counter is maintained in step with the time display. The state of the synchronous counter is compared with that of the reference counter in a comparator circuit which acts, in the case of a deviation therebetween, to govern the number of pulses per unit of time applied through the logic circuit to the stepping motor and to the synchronous counter to effect the necessary correction in the displayed time.
Thus with a fast correction system as disclosed in the above-noted patents, it is no longer necessary, where a small deviation exists in the time display, to first operate the crown of the watch to arrest the seconds hand at its zero position (12 o'clock) and then wait to hear a reference time signal (radio or telephone) before again operating the crown to restart the watch. The prior art fast correction system is intended to simplify the time setting of the watch, for it is only necessary when hearing the time reference signal to momentarily operate the actuating element or button to automatically bring the seconds display into step with the time reference signal without any additional operations or expedients.
In principle, it is possible with the correction arrangement disclosed in the above-noted patents, after resetting the reference counter, to supply by means of the comparator circuit additional pulses from the divider to the synchronous counter and to the display to correct for a minus deviation in the seconds reading or to inhibit the transmission of pulses from the divider to the synchronous counter and to the display to correct for a plus deviation.
The prior art correction system is intended to take care of relatively small deviations of plus or minus 30 seconds from precise time, this being sufficient for modern electronic watches, particularly those which include a quartz-crystal frequency standard whose monthly rates usually remain within these tolerances. The great advantage of this fast correction system is that the user can make the necessary correction not more than once a week or even less frequently, simply by operating the actuating element without having to stop and restart the watch.
In practice, however, the prior art correction arrangement outlined above does not result in a well-functioning, trouble-free device. Indeed, handsetting of the watch can give rise to inaccuracies. For example, when, after setting the hands, the crown is pushed in to restart the watch, the resumption of the time display will be off by one second; for the stepping motor must await the next seconds pulse from the last stage of the frequency divider. Such inaccuracies are unacceptable for a quartz crystal electronic watch or any other highly accurate timepiece.